1. Field of the Invention
The present invention relates to a two-component developing agent for developing an electrostatic latent image and a method for forming an electrophotographic image. More specifically, the present invention relates to a two-component developing agent that is used for developing an electrostatic image, causes no fogging and toner scattering and provides high-quality images even in hot and humid conditions, and a method for forming an electrophotographic image using the two-component developing agent.
2. Description of Related Art
In recent years, electrophotography has been used in a field of commercial printing and required to always be able to stably provide high-quality images. To meet such a requirement, it is need to use a two-component developing agent that has high durability and is difficult to be deteriorated due to changes in temperature and humidity and/or printing of a large number of sheets. Given this need, there are provided carrier particles composed of carrier core particles and a hydrophobic resin coat, e.g., a silicone resin coat, on the surface of the carrier core particles.
A silicone resin coat on the carrier core particles is hard and thus difficult to be worn. Therefore, when a “toner spent problem” (this means adhesion of fragments of toner particles, an exterior additive of toner particles and/or a component of the toner base particles to the surface of carrier particles) occurs on a silicone resin coat of carrier particles, the degree of the “toner spent problem” gradually increases, and chargeability of the carrier particles is deteriorated.
To avoid accumulation of a “toner spent problem”, it is preferable to use a coating resin moderately easy to be worn so that the outer surface of the resin coat can be refreshed (“refresh” in this context means that the resin surface that is not deteriorated by the “toner spent problem” and fresh becomes exposed). Such a resin can be exemplified by a copolymer resin of an alicyclic methacrylate monomer and a linear methacrylate monomer described in Japanese Patent No. 3691085.
As a resin for avoiding the “toner spent problem” to the surface of carrier particles, Japanese Patent Application Laid-Open Publication No. Hei7-219280 describes a resin that has small adhesion force to toner particles and large water contact angle, and is obtained by polymerizing an isobutyl methacrylate monomer in a non-aquatic condition.
On the other hand, to obtain high-resolution and high-quality images, a chemical toner which is composed of small spherical particles and is produced by suspension polymerization or emulsion polymerization has recently been used. A chemical toner is produced in an aqueous medium and thus easy to absorb water. Hence, a chemical toner is characterized in that a change in the amount of charge of the toner based on changes in environmental temperature and humidity is larger than that of a toner produced by crushing, and thus a chemical toner has high environmental dependency. Because of the characteristics, even carrier particles coated with the resin produced by polymerizing an isobutyl methacrylate monomer are used, the amount of charge of a toner largely changes depending on changes in environmental temperature and humidity.